(1) Field of the Invention
The present invention generally relates to an evaporative fuel purge apparatus, and more particularly to an evaporative fuel purge apparatus for an internal combustion engine in which fuel vapor evaporated in a fuel tank is stored in a canister and the fuel stored in the canister is fed into an intake passage of the engine.
(2) Description of the Related Art
In an evaporative fuel purge apparatus, fuel vapor, evaporated in a fuel tank, is fed into a canister containing activated carbon, the fuel vapor being adsorbed by the activated carbon of the canister so that the fuel vapor is stored in the canister, thus preventing the fuel vapor from escaping to the atmosphere. A purge passage connecting the canister and an intake passage of an internal combustion engine is provided in the evaporative fuel purge apparatus so that the stored fuel vapor is purged into the intake passage of the engine through the purge passage. A mixture of intake air and the fuel vapor in the intake passage is supplied to a combustion chamber of the engine via an intake value.
A flow rate of intake air into the intake passage is normally low when the engine is in an idling condition. Therefore, if the fuel stored in the canister is freely purged into the intake passage when the engine is in such a condition, it is difficult to maintain the stable operation of the engine. For example, the driveability deteriorates and the engine may stall. In order to eliminate this problem, a control valve is provided in the purge passage to control a flow of the fuel vapor being fed from the canister into the intake passage in response to the operating condition of the engine. However, if the control valve malfunctions, it is difficult to suitably control the flow of the fuel vapor from the canister to the intake passage to ensure the stable operation of the engine. Therefore, it is desirable that the evaporative fuel purge apparatus be provided with a safety unit for taking necessary measures when the control valve malfunctions.
In the prior art, there is an evaporative fuel purge device having a safety unit against a malfunction of a control valve in a purge passage. For example, Japanese Laid-Open Utility Model Application No.2-61173 discloses such a device. In this conventional device, a control valve is mounted in a purge passage connecting a canister and an intake passage for controlling a flow of fuel vapor being purged from the canister into the intake passage. In the purge passage downstream of the control valve, a diaphragm valve is provided. This diaphragm valve functions to close the purge passage when a throttle valve in the intake passage is switched to its closing position. If the control valve malfunctions and stops operating when it is still at its open position, the purge passage to the intake passage remains open. In such a case, the diaphragm valve is switched to its closing position, so that the purge passage is closed and the fuel vapor in the canister is not fed into the intake passage through the purge passage when the control valve malfunctions.
In the above mentioned evaporative fuel purge device, the function of the diaphragm valve is effective only when the control valve malfunctions and the purge passage to the intake passage remains open. However, if the control valve malfunctions when it is at its closing position, the purge passage is closed by the control valve and the fuel vapor is continuously supplied from the fuel tank to the canister, the canister finally overflowing since the quantity of fuel that can be stored in the canister is limited due to the capacity of the canister, so that excessive fuel vapor may escape from an opening of the canister to the atmosphere. Therefore, it is desirable to provide an evaporative fuel purge device having a safety unit that effectively functions even if the control valve malfunctions and the purge passage is closed.